1. Field of the Invention
The present invention relates to a vulcanizable rubber composition suitable for the production of rubber materials having excellent scorching stability and heat aging resistance.
2. Description of the Prior Art
The heat aging resistance of rubber material has hitherto been improved by using a small amount of sulfur and a vulcanizing accelerator, such as benzothiazylsulfenamide, in an amount of more than the amount to be ordinarily used as a vulcanizing accelerator. However, such system is slow in the vulcanization velocity and further is apt to contaminate the resulting vulcanized rubber. Furthermore, it is well known that in order to attain the above described object, sulfur is wholly or partly replaced with sulfur donor or organic sulfur compound which can be reacted with rubber. For example, a method, wherein rubber is vulcanized by using tetramethylthiuram disulfide or dithiol without using sulfur, has hitherto been proposed. However, these compounds are apt to cause premature cure, scorching or bloom in the processing and molding steps of rubber. Among the conventional organic sulfur donors, N,N'-dithiomorpholine is most effective against premature curing and scorching. However, the vulcanized rubber composition containing N,N'-dithiomorpholine still has various problems to be improved. The N,N'-dithiomorpholine is represented by the chemical formula ##STR1## while, the sulfenamide compound according to the present invention has a group X selected from the group consisting of residues of hydrocarbon, alkyl ether and secondary alcohol having 2-20 carbon atoms and residues represented by the general formulae ##STR2## between adjacent two sulfur atoms, and typical compound of the sulfenamide compound is, for example, 1,4-bis(morpholinothio)benzene having a chemical formula ##STR3## It has never hitherto been known that such sulfenamide compound is effective as a vulcanizing agent, a vulcanizing accelerator or a stabilizer.